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PostPosted: Fri Jan 19, 2024 8:58 am 
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1275cc
1275cc
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Joined: Sun Apr 29, 2012 1:19 pm
Posts: 4315
Location: Wollongong, NSW
jpodge wrote:
Nice work there Tim. Am I reading correctly that you saw better performance in the Dellorto vice the twin carbs? How does the HS4 stack up against the dellorto?

While you are still troubleshooting, I think you've best going with a double head gasket to see if reducing the CR is what the engine is wanting - cheap and quick way to diagnose. If it's happy there, then completely agree with getting a few cc from grinding the beaks down on the cylinder head. You've definitely got the skills to do that job with a basic rotary tool and a course sanding bit.

The carbs are a bit of a long story Joe :lol:

The timeline is something like this:
1) Dellorto DHLA40 with 32mm chokes. This ran perfectly at the time and set the benchmark
2) Twin HS2 on a Cooper S manifold. The performance was close enough I couldn't tell the difference
3) Dellorto DHLA40 with 34mm chokes. This has had the best performance so far
4) Dual HS2 on the Peter Manton manifold. I chose these as they were used on the sports 850 and was available as an upgrade kit in the 60's. The manifold appears to have been designed for an 850 engine and the ports are absolutely tiny, even with some clean up with a sanding tool in the dremel it's still not 1" diameter all the way through. I'm a bit hesitant to bore it right out for better performance and ruin it

Generally speaking, the Dellorto is much louder, sounds better, sharper throttle response. The SU carbs are much smoother and quieter.

Next/future options are:
5) HIF44 single carb (which is 1.75" diameter). These are the go-to carb for anything 1275 in the UK and a more modern alternative to the HS6. The single carb definitely appeals and it should be more than sufficient for my engine
6) Twin H4 carbs (which are 1.5" diameter). These were used in the 60's on racing minis as they bolt straight up to the standard manifold. These just need a clean and reassembly
7) Dellorto DHLA40 with 34mm chokes after a rebuild

Good point about the double head gasket too. It is the cheapest option and easily reversible so worth a shot

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